High voltage electrical distribution systems utilize connectors, commonly known as "loadbreak elbows", at junctures of the conductors transmitting the current. Such a connector typically comprises a conductor surrounded by an insulating layer, a semiconductive layer and then an insulating sheath, all encased in a semiconductive outer shield, and includes a test point terminal embedded in the insulating sheath and exposed for contact from outside of the shield. A voltage on the conductor capacitively couples a first voltage on the test point terminal and a second voltage on the outer shield.
Service personnel commonly encounter difficulty in reliably determining whether or not a voltage is present on a loadbreak elbow. This is of considerable importance, since the safety of service personnel effecting service on such a system may depend upon the reliability of a status indicator correctly indicating the status of the connector to prevent electrical shock hazards.
A variety of indicating devices for such purpose are known. Capacitance indicators, typically a length of insulating tube with a metal probe at one end and a metal handle at the other, are designed to detect a voltage difference between the user's body and the object being tested. This type of device often has the disadvantage of requiring a power source independent of the conductor to drive the sensing mechanism, and further requires direct contact with the energized conductor. Moreover, the capacitance between the user's hand and the probe handle or the user and ground, being part of the measuring circuit, can affect the instrument's sensitivity.
Electronic indicators are the most commonly used indicators. An alternating electric field is detected, and the voltage reading is compared with a reference voltage. If the difference between the detected voltage and the reference voltage exceeds a preselected value, a battery powered indicator is activated. These devices require a power source independent of the conductor and further must be carefully calibrated. They are subject to errors from stray electric fields, and are usually complex and expensive.
Resistance indicators essentially comprise a voltmeter with one terminal grounded, which detects a voltage by direct contact with a high voltage-line. These devices, like capacitance indicators, must be carefully employed in order to avoid electrical shock, and draw a current from the conductor being tested which can affect the voltage reading.
All of the indicating devices mentioned above must be transported with service personnel, and must have associated a self test feature or some other means of indicating to service personnel that the indicating device is operational. In the absence of such a feature, failure of the device could indicate a false voltage status which may lead service personnel to assume that there is no voltage on the conductor when a voltage is in fact present, which presents an obvious safety hazard. However, the provision of a self test feature in indicating devices which do not operate from a power source external to the conductor is difficult and requires additional circuitry which renders these devices more complex and bulky.